1. Field of the Invention
The present invention relates to a speed control apparatus which controls speed such as rotating speed and response speed of an AC (alternating current) motor, for example, of an induction motor, synchronous motor and AC commutator motor, etc.
2. Description of the Prior Art
In various industrial fields, motors of various scales and kinds are used in general and these motors can be broadly divided into DC (direct current) motors and AC motors. Of these various kinds of motors, the DC motors are very excellent in the controllability assuring high reliability of operation but inferior in applicability due to complicated structure and high manufacturing cost. Therefore, the AC motor is frequently used in many industrial fields. AC motors are classified, for example, induction motors, synchronous motors and AC commutator motors, etc.
The speed control of an AC motor has been conducted with a speed control apparatus as indicated in the block connection diagram of FIG. 1. As shown in FIG. 1, this speed control apparatus is composed of a speed compensator 1, current compensators 2 and 3, a first coordinate transformer 4 which transforms an input current detected of an AC motor into magnetic flux and torque as described later, a slip frequency calculator 5 which calculates a slip frequency of torque current, an adder 6 which calculates the addition of the calculated value of this slip frequency and the detected speed value of the AC motor, an integrator 7 which integrates outputs of the adder 6, a second coordinate transformer 8 which transforms outputs of the current compensators 2, 3 to a 3-phase AC voltage command, a comparator circuit 9 which receives the output of the second coordinate transformer 8 as an output, a dead-time signal forming circuit which delays timing of an output of the comparator circuit 9 for a certain period, a basic amplifier 11 which insulates outputs of such dead-time signal forming circuit 10 and amplifies them, an inverter 12 utilizing a semiconductor element for amplifying power such as a power a transistor a which is responsive to the output of base amplifier 11 and controls a drive current of the AC motor in vector, a current detector 13 which detects an AC motor drive current to be input to the first coordinate transformer, an induction motor 14 as an AC motor to be driven through the vector control, and a speed detector 15 which detects rotating speed of the induction motor 14. In FIG. 1, the speed control apparatus is illustrated, except for the elements such as a compensating means, for example, inverse electromotive force compensating means which is not directly related to explanation of the present invention.
Operations based on above structure are explained hereunder.
As is well known, vector control is intended to attain a high response control by individually controlling the current component for magnetic flux and the current component for torque of the current supplied to the motor. Therefore, a coordinate which rotates in synchronization with the secondary magnetic flux is considered assuming the axis component i.sub.d (current for magnetic flux) which is parallel to the secondary magnetic flux and the axis component i.sub.q (current for torque) which is orthogonal to said secondary magnetic flux as the control object.
In FIG. 1, deviation ".omega..sub.r *-.omega..sub.r " of a speed command .omega..sub.r * and a speed detection signal .omega..sub.r from the speed detector 15 is amplified by the speed compensator 1 and its output becomes a torque current command i.sub.q *. Meanwhile, a 3-phase AC current detected by the current detector 13 is transformed to a magnetic flux current i.sub.d and a torque current i.sub.q by the first coordinate transformer 4. Therefore, deviation between said torque current command i.sub.q * and torque current i.sub.q is amplified by the current compensator 2 and becomes a q-axis voltage command v.sub.q *. It is then input to the second coordinate transformer 8. In the same way, a magnetic flux current command i.sub.d * is set in accordance with the characteristic of load motor and deviation between the magnetic flux current command i.sub.d * and magnetic flux current i.sub.d is amplified by the current compensator 3 and becomes a d-axis voltage command v.sub.d *. The q-axis voltage command v.sub.q * and the d-axis voltage command v.sub.d * are then input to the second coordinate transformer 8. In the second coordinate transformer 8, the d-axis voltage command v.sub.d * and the q-axis voltage command v.sub.q * are converted to the 3-phase AC voltage commands v.sub.u *, v.sub.v *, v.sub.w * and these are input to the comparator circuit 9. These are compared therein with a triangle wave signal and thereby the ON-OFF signal of the transistors forming a transistor inverter 12 is generated. However, since a certain time delay is generated during transfer between ON and OFF states of the transistors, the ON timing is delayed for a constant period T.sub.d so that a pair of transistors connected in series between the DC buses are not turned ON simultaneously in the transistor inverter 12. The dead-time signal forming circuit 10 is provided for this purpose. An output of this circuit is amplified and insulated by the basic amplifier 11 as an actual drive signal for the transistor inverter.
Moreover, a slip frequency .omega..sub.s must be controlled by the well known method utilizing the following equation in order to synchronously rotate the control coordinate axis with the secondary magnetic flux. EQU .omega..sub.s =(R.sub.2 i.sub.q)/(L.sub.2 i.sub.d)
Where, R.sub.2 is a secondary resistance, L.sub.2 is a secondary inductance, respectively.
The slip frequency calculator 5 calculates a slip frequency .omega..sub.s using above equation. The slip frequency .omega..sub.s output from this calculator 5 and the speed detection signal .omega..sub.r detected by said speed detector 15 are added in the adder 6. A position data .theta. of secondary magnetic flux can be obtained by integrating such added output in the integrator 7 and this position data .theta. is output to the first coordinate transformer 4 and the second coordinate transformer 8, thus completing the coordinate transformation.
With the structure and operations described above, a drive current is transformed into a magnetic flux current and a torque current and is controlled as a DC component according to the speed control apparatus of the AC motor of the prior art. Therefore, rotating speed control of AC motor can be realized easily and digital control utilizing a microprocessor, etc. can also be realized easily. In this point, this apparatus can be said to be a very effective control apparatus.
However, the existing speed control apparatus for an AC motor has the following problems.
The input side of transistor inverter 12 is provided with the dead-time signal forming circuit 10 in order to prevent a pair of transistors connected in series from being turned ON simultaneously. This dead time makes non-linear i.sub.d /v.sub.d *, i.sub.q /v.sub.q * and generates an offset as shown in FIG. 3 and as explained later. The current compensators 2 and 3 carry out the proportional and integral compensation (PI compensation) and influence of the non-linear characteristic has been controlled to a minimum condition by raising a proportional gain. However excessively higher gain makes the margin of stability of the control system small. Particularly, when the sampling control is carried out using a microcomputer, the allowable upper limit of gain is lowered by other restrictions and therefore compensation is insufficient and there arises a problem in the region where a voltage amplitude is so low that a loop gain of the current control system is equivalently lowered, response of current control system is also lowered and as a result sufficient speed response cannot be obtained.